The present invention relates to a thin film semiconductor device manufactured by a laser annealing method of scanning a laser beam having a rectangular cross-section shape and a method of manufacturing the device.
In recent years, liquid crystal display devices employing a thin film semiconductor device constructed of polycrystalline silicon thin film transistors and so on, in which a scan line drive circuit and a data line drive circuit can be integrally formed on a glass substrate together with transistors for driving pixels, have been developed technologically and commercially as a promising technique for a reduction in size, an increase in fineness and a reduction in cost of the liquid crystal display devices.
In particular, the laser annealing method which is a means for forming a polycrystalline silicon film on glass substrate can form a polycrystalline silicon film of a large area by scanning a spot-shaped or rectangle-shaped laser beam, and therefore, the method receives attention as a mass-production technique.
FIGS. 11A through 11E show a method of manufacturing a polycrystalline silicon thin film transistor by the laser annealing method.
First, in FIG. 11A, a protective film 101 of a silicon oxide film or the like is formed on a glass substrate 100 (of about 300 mm to 1000 mm square) and thereafter an amorphous silicon film 102 is formed on the protective film 101.
Next, as shown in FIG. 11B, a laser beam 121 is irradiated from a laser light source 120 to the amorphous silicon film 102 thereby annealing the amorphous silicon film 102, so that the amorphous silicon film 102 is crystallized to be formed into a polycrystalline silicon film 103. In the case of a continuous oscillation type argon ion laser, the laser beam 121 has a spot-like shape of a diameter of about 100 .mu.m. In the case of a pulse oscillation type excimer laser having an optical system like a beam expander, the beam shape is a rectangular shape having a shorter side of 0.1 to several millimeters and a longer side of 100 to several hundred millimeters. By scanning the laser beam 121, the polycrystalline silicon film 103 is obtained in a wide area on the glass substrate 100.
Next, as shown in FIG. 11C, the polycrystalline silicon film 103 is patterned by a photolithographic method, so that an island-shaped silicon layer 104 is formed.
Next, as shown in FIG. 11D, an oxide silicon film 105 is formed on the protective film 101 and the silicon layer 104 and made to serve as a gate insulating film.
Subsequently, as shown in FIG. 11E, a gate electrode 106, a source region 107 and a drain region 108 containing a high concentration of impurities, an interlayer insulating film 109, a source electrode 110 and a drain electrode 111 are formed. The gate electrode 106, source region 107, drain region 108, source electrode 110 and drain electrode 111 constitute a thin film transistor.
FIG. 12 shows a circuit-integrated type liquid crystal display device employing the above thin film transistors. In FIG. 12, a plurality of pixel electrodes 201 arranged in a matrix form are connected to respective switching transistors 202, and the switching transistors 202 are sequentially turned on and off by a signal on a scan line 203, thereby writing image data inputted to a data line 204 into the pixel electrode 201 when the switching transistor 202 is turned on. A data line drive circuit for driving the data line 204 is constructed of an amplifier circuit 205, which could be omitted especially in the case of small panels below the display size of about 5 inches diagonal, an analog switch 206 and a logic circuit 207 for controlling the turning-on and -off of the analog switch 206. A scan line drive circuit for driving the scan line is a selection pulse generator circuit 208 for successive selection of the scan line 203.
The glass substrate on which the pixel electrodes 201 connected to the switching transistors 202 shown in FIG. 12 are arranged in a matrix form and another glass substrate (not shown) on which opposite transparent electrodes are formed are bonded together with a gap of several micrometers interposed between them after conducting an alignment process of the inside surfaces of both the glass substrates, and then a liquid crystal material is infused into this gap, so that a liquid crystal display device is obtained.
In regard to a drive-circuit-integrated active matrix type liquid crystal display device in which a polycrystalline silicon film formed by the laser annealing is used for transistors, an arrangement construction and a manufacturing method of the transistors are disclosed in the document of Japanese Patent Laid-Open Publication No. HEI 7-92501.
The essence of the invention disclosed in this document of Japanese Patent Laid-Open Publication No. HEI 7-92501 is to arrange in a linear form the transistors of the scan line drive circuit and the data line drive circuit in an attempt at reducing the time required for laser annealing. Further, by arranging the pixel electrode use transistors on a line extended from the transistors in the scan line drive circuit or the data line drive circuit, the laser annealing for forming a polycrystalline silicon film which will serve as an active layer of these transistors can be concurrently executed. Further, assuming that a scan pitch of the laser beam is Pt, then a relation between the scan pitch Pt and a stripe width of a belt-shaped polycrystalline silicon film obtained by the laser annealing scan is set so that Pt&gt;(stripe width.times.2).
However, when scanning a laser beam having a rectangular cross-section shape, to satisfy the condition of scan pitch Pt&gt;(stripe width.times.2) disclosed in the aforementioned document of Japanese Patent Laid-Open Publication No. HEI 7-92501 is not compatible with the effect of reducing the defect density in crystal grain size due to overstriking and the effect of reducing the variation in distribution of the crystalline state attributed to the variation in intensity of the laser beam shot in each time, and this leads to a problem that the transistor characteristics are impaired. Furthermore, the document of Japanese Patent Laid-Open Publication No. HEI 7-92501 mentions no concrete method of aligning in position the rectangular polycrystalline stripe width with the polycrystalline silicon layer of the transistor, and it is very hard to put the polycrystalline silicon layer of the transistor into the stripe width of the polycrystalline silicon film.
Comparing each other the aforementioned two types of laser annealing methods, i.e., the method of using a spot-like beam shape as observed in the case of the continuous oscillation type argon ion laser and the method of using a rectangular beam shape as observed in the case of the pulse oscillation type excimer laser, the former method requires a complicated optical system for scanning the spot-shaped laser beam and is hard to uniformly effect the annealing on the glass substrate of several hundred millimeters square. Therefore, the latter method of using a pulse oscillation type excimer laser having a rectangular beam shape is advantageous in terms of the mass-production technique.
For the above reasons, the latter laser annealing method of scanning a laser beam having a rectangular cross-section shape is generally adopted. This rectangular beam width is about several hundred micrometers to several millimeters, and the scan pitch is about several ten micrometers to several hundred micrometers, providing an overlap shot (irradiation) region in the scan stage. This is because the effect of reducing the defect density in crystal grain size and the effect of reducing the variation in distribution of the crystalline state attributed to the variation in intensity of each laser beam shot are produced by executing multi-time overstriking on one silicon region.
However, the transistor characteristic variation occurs even when forming a plurality of polycrystalline silicon thin film transistors on a glass substrate by the method of executing scan through the overstriking of a laser beam having a rectangular cross-section shape. This is mainly ascribed to the variation of defect density of the polycrystalline silicon film and the nonuniformity in crystal grain size, and a difference of thermal melting and cooling processes in the laser annealing stage exerts a great influence on the defect density and the crystal grain size of the polycrystalline silicon film. Furthermore, the bottom cause is estimated to be ascribed to the beam shape and beam output of the laser beam or a variation in beam locus distribution in the scan stage.
A mechanism of causing a nonuniformity in the polycrystalline silicon film by the laser annealing method of executing scan through the overstriking of a laser beam having a rectangular cross-section shape will be described with reference to the models shown in FIG. 13 and FIG. 14. FIG. 13 shows a relation between a distance in the beam width direction and a laser power, wherein the laser power has an inclination of gradually reducing outward on both sides of the laser beam. In this inclined region, a critical power serving as an energy required for melting the silicon exists, and the region not lower than this critical power becomes effective in crystallization. It is now assumed that effective regions having this inclination are A1 and A2 and a center region in which the laser power is relatively stable is B, and a case where a laser beam 211 is made to scan in an overstriking manner at a scan pitch C on an amorphous silicon film 210 as shown in FIG. 14 is considered. In this case, an .alpha.-region formed initially by the application of the A2 region of the laser beam and .beta.-region formed by the application of the B region of the laser beam are generated on the polycrystalline silicon film formed by annealing. In general, the higher the laser power is, the greater the crystal grain size of the resulting polycrystalline silicon film is. Therefore, the crystal grain size in the .alpha.-region becomes smaller than that of the .beta.-region. If the polycrystalline silicon film initially formed are subsequently subjected again to the annealing through the overstriking of the laser beam, the effective laser power reduces since the polycrystalline silicon state has a smaller laser beam absorption efficiency than that of the amorphous state, so that no significant variation in crystal grain size occurs and the relation in crystal grain size between the .alpha.-region and the .beta.-region does not change.
As described above, the regions having different crystal grain sizes are distributed in the polycrystalline silicon film formed by the laser annealing method of executing scan through the overstriking of a laser beam having a rectangular cross-section shape. Furthermore, these regions are changed by the mechanical fluctuations of the laser annealing apparatus, i.e., a fluctuation in laser power occurring every shot, a fluctuation in beam cross-section shape occurring every shot as shown in FIG. 13 or a fluctuation in scan pitch. The above factors cause a problem that the characteristics of the semiconductor elements which use the polycrystalline silicon film formed by the laser annealing method as the active layer vary.